A large number of people suffer from health conditions which are either directly or indirectly related to their cardiac function. For example, hypertension refers to a chronic disease in which the patient's systemic blood pressure is elevated above that which is considered a healthy level. As a chronic condition, hypertension can contribute to a wide variety of well known health ailments. Accordingly, therapy for a known hypertension condition is typically prescribed to ameliorate the hypertension condition and related effects. Depending on the severity of the hypertension as well as the patient's individual condition, typical therapies can include diet modification, an exercise regimen, and/or a medication regimen.
Another category of cardiac related health ailments include cardiac arrhythmias. A number of medications are known which can help restore, at least partially, more normal sinus rhythm for certain patients. Another known category of therapy for cardiac arrhythmias includes implantable cardiac stimulation devices, such as pacemakers and/or implantable cardioverter-defibrillators (ICDs). Implantable cardiac stimulation devices automatically monitor the ongoing cardiac activity of the patient and selectively provide direct electrical stimulation to the cardiac tissue when indicated in an attempt to restore the patient's intrinsic rhythm to a sinus rhythm or alternatively to substitute for the patient's intrinsic rhythm and assume control of the heart's activity for at least some period of time, such as in pacing. While implantable cardiac stimulation devices can be quite effective in treating cardiac arrhythmias, they are not generally considered appropriate therapy for patients with hypertension but without a history of cardiac arrhythmias.
A drawback to the use of implantable cardiac stimulation devices, such as pacemakers, as therapeutic interventions for cardiac related health ailments, such as cardiac arrhythmias, is that they are relatively crude in nature as compared to natural cardiac pacing in that they directly provide electrical stimulation to one or more chambers of the heart (atria/ventricles). This direct electrical stimulation acts as a surrogate for the native electro-chemically based natural pacing functions of the heart, however, this artificial electrical stimulation does not fully replicate the natural intrinsic stimuli which induce activity in the cardiac tissue.
The use of implantable cardiac stimulation devices for some patients can lead to a wide variety of symptoms known generally as “pacemaker syndrome.” Pacemaker syndrome refers broadly to adverse hemodynamic and/or electro physiologic consequences associated with the presence of the artificial direct electrical stimulation in place of the natural intrinsic pacing of the patient. Pacemaker syndrome may be found on an intermittent or a persistent basis, and the severity of the symptoms can range from relatively minor to more severe, and can also be considered to include limitations on the patient's ability to achieve an otherwise optimal status. Pacemaker syndrome is frequently associated with a loss of appropriate atrial-ventricular (AV) synchrony, however, can also occur due to an inappropriate AV interval. Pacemaker syndrome can also arise from inappropriate rate modulation which refers to the modulation of the cardiac output, including heart rate, based on the patient's varying metabolic need.
Implantable cardiac stimulation devices provide therapy by detecting and discriminating relatively low amplitude complex time varying electrical signals from one or more heart chambers, evaluating these signals as well as other indications of the patient's physiological status, and delivering appropriate therapy to one or more chambers of the heart. Accordingly, implantable cardiac stimulation devices are relatively complex and expensive to design and produce. Their implantation also requires the expertise of highly trained and skilled clinicians as well as for periodic follow-ups to monitor the therapy and the patient's response for “fine-tuning” the device settings. Thus, implantable cardiac stimulation devices are a relatively costly therapy to provide and maintain both in terms of the devices themselves and the services of trained clinicians.
One possible approach to provide therapy while avoiding the physiologic and cost drawbacks of direct electrical stimulation of the cardiac tissue would be to provide some manner of indirect therapy or stimulation, such as provided by the body's natural feedback mechanisms. For example, providing stimulation to the vagus and/or sympathetic nerves has been considered as a possible avenue for therapy delivery. However, it has as yet proven impractical to access these nerves and to realize effective placement of leads on them to provide stimulation on a long term basis.
Thus, it will be understood that there is an ongoing need for providing interventional therapy for patient conditions, such as cardiac arrhythmia and/or hypertension, in a manner which reduces the negative consequences or side effects of known therapies. There is a desire for a therapy option which avoids the negative consequences of direct electrical stimulation of the cardiac tissue, such as the aforementioned pacemaker syndrome, as well as undesirable side effects attendant pharmacological therapies, such as adrenergic and/or anti-hypertensive drugs. There is also a desire for therapy which is less costly and complex to implement and also for therapy which more closely emulates natural physiological feedback and response.